This invention relates generally to sealing compounds for pipe joints and the like and more particularly to a fluorocarbon based sealing compound in the form of a thixotropic paste capable of sealing any threaded joint and suitable for oxygen and food service.
To prevent or minimize leakage of a fluid through mechanical clearances use is made of a packing material. Where the clearance is that existing between the threaded end of a pipe received within an internally-threaded pipe joint or fitting, it is the common practice to apply a sealing compound of "dope" to the threads of the pipe before it is screwed into the joint or fitting.
One common type of sealing compound for this purpose is formed by a mixture of linseed or mineral oil, plasticizers and fine clay particles. These widely used compounds have a number of disadvantages; for when the oil dries out, the sealant cakes and becomes less effective. Also, should the pipe coupling rust, it may be difficult or impossible to disassemble the joint.
Another commercially available type of sealing compound makes use of fine asbestos particles as a filler, but this is objectionable because of the hazardous aspect of asbestos. Some sealing compounds incorporate granular particles of "Teflon" in an oil carrier to improve the lubricity of the compound. However, such compounds are subject to oxidation, volatilization and other drawbacks.
In recent years, pipe thread sealant tapes have become commercially available, such tapes being made from an extruded, unsintered Teflon TFE-fluorocarbon resin. This tape, which is non-flammable, non-toxic, and self-lubricating, is intended as a replacement for dopes to seal threaded connections on pipes made of plastic, iron and steel, copper, brass, aluminum and other materials. Because Teflon has a unique combination of electrical, chemical, temperature and low-friction properties (see U.S. Pat. No. 2,230,654), it may be used as a pipe sealant in environments which preclude conventional dopes, such as in hard-to-deal gas or liquid lines employed in the chemical and petrochemical industries, in lines carrying liquid oxygen, nitric and other acids, or in lines conducting caustics and other corrosives.
Unlike liquid or paste dopes that burn, spill or cake, a tape sealant is easy to use, for one merely lays the end of the tape on the pipe, starting near the end, the tape then being wrapped around in the direction of the threads. With a thread sealant tape formed of Teflon, threaded pipe joints will not seize and they can be disassembled easily. This is important in threaded joints requiring subsequent disassembly, such as those included in test gauges and in heating, air conditioning and fuel systems.
While Teflon tape has many advantages over standard sealing compounds or dopes, it has several drawbacks, some of which are relatively serious. Unless the tape is wrapped in the proper direction about the pipe threads, it will unravel when the joint is run up. Also, if the pipe end is improperly covered, tramp fragments of tape will get into the system and may plug orifices and valves. Because the tape is fabricated of pure Teflon, it has a strong tendency to cold flow, producing clearances, causing leakage. Moreover, when a taped joint is disassembled, the tape unravels and must be picked off the threads for remake.
In my prior U.S. Pat. No. 3,879,302 there is disclosed a sealing compound which overcomes many of the drawbacks inherent in known types of sealing compounds or Teflon tapes. The sealing compound disclosed in my prior patent is formulated from particles of inert filler material intermingled with an aqueous dispersion of microfine fluorocarbon powders, the dispersion being stabilized by a charge-neutralizing agent such as silane, which prevents clotting of the powders.
While my prior patented compound is useful in a number of situations, there are certain applications where it is not acceptable, as in food processing plants. It is essential for food processing that the pipe sealant for the plumbing be non-contaminating and physiologically inert. Also it is important in this and in other applications that the compound even though subject to freezing, have freeze-thaw stability so that its properties are unaffected by repeated cycles of freezing and thawing. While the use of silanes to impart freeze-thaw characteristics to a sealing compound is acceptable in many applications, its use is objectionable in food processing environment.